This invention relates to the field of catalytic conversion of hydrocarbons in a fluidized bed reaction zone. More particularly, the invention relates to directly transferring heat to a catalytic aromatization or dehydrogenation reaction zone by circulating catalyst, inert particles or both through a combustion zone and then through a catalytic reaction zone.
Heat transfer via inert particles has been used in catalytic conversion processes. For example, U.S. Pat. No. 2,763,596 to Feldbauer et al. describes reforming in a mixture of catalyst and inert heat transfer solid, with a solids elutriation step. Feldbauer is, however, directed to the reforming of a naphtha range feed. In contrast, the present invention is directed to the dehyrogenation or aromatization of a C.sub.2 -C.sub.5 aliphatic stream.
U.S. Pat. No. 2,913,392 to Richards describes a process which combines a first step of catalytic cracking with a second step of noncatalytic thermal cracking. Product from the first step is subjected to noncatalytic thermal cracking in the presence of inert solids.
U.S. Pat. No. 2,763,595 to Fritz describes a reforming process in which the catalyst regeneration reaction is used to heat circulating inert solids. No suggestion is made of using a supplemental fuel source.
U.S. Pat. No. 2,763,597 to Martin et al. discloses a reforming process in which heat evolved from catalyst regeneration is transferred to the reactor by a mixture of circulating catalyst and inert solids. The Martin reference makes no mention of using supplemental fuel and additionally uses steam to strip product from the mixture of inert solids and catalyst. Contact with water vapor at elevated temperatures causes steaming deactivation of many catalysts useful for aromatization.
Unlike a fluidized catalytic cracking (FCC) process, the fluidized bed catalytic aromatization of an aliphatic feedstream does not generally operate in a heat balanced mode. That is to say that in the operation of a heat balanced process, e.g. FCC, the thermal energy released during regeneration by the combustion of coke deposited on the catalyst equals or exceeds the endothermic heat of reaction required for the conversion step. In contrast, catalytic aromatization of C.sub.2 -C.sub.5 aliphatic hydrocarbons does not produce sufficient coke to supply its own endothermic heat of reaction. Consequently, design routes previously favored were moving- or fixed-bed operation using a fired heater and fluidized bed operation using an external process heat source. As can be seen by one skilled in the art, these designs have inherent limitations. Preheating the feedstock to reaction temperature in a fired heater causes undesirable side reactions including the formation of coke in the heater tubes. Fixed bed reactors require periodic regeneration and fluidized bed reactors using a second process unit as a heat source require close proximity of process streams available for cooling at initial temperatures above 1000.degree. F. Further, preheating catalyst to high temperatures to supply the heat of reaction in a fluidized-bed system has been found to accelerate catalyst deactivation.